Conventional gypsum wallboard or panel is typically manufactured from a plaster slurry wherein a wet slurry of calcium sulfate hemihydrate, generally referred to as calcined gypsum, is placed between two layers of paper and the slurry is allowed to set. The set gypsum is a hard and rigid product obtained when the calcined gypsum reacts with water to form calcium sulfate dihydrate. Gypsum is calcium sulfate in the stable dihydrate state, CaSO4.2H2O, and includes the naturally occurring mineral, the synthetically derived mineral, and the dihydrate material formed by the hydration of calcined gypsum. Calcined gypsum is either calcium sulfate hemihydrate (CaSO4.½H2O) or calcium sulfate anhydrite (CaSO4). When calcium sulfate dihydrate is heated sufficiently, in a process called calcining, the water of hydration is driven off and there can be formed either calcium sulfate hemihydrate or calcium sulfate anhydrite, depending on the temperature and duration of exposure. When the dihydrate is heated sufficiently in a saturated steam environment, the dihydrate dissolves and the hemihydrate form precipitates out of solution as well formed crystals. When water is added to the calcined gypsum to cause the gypsum to set, in essence, the calcined gypsum reacts with water, and the gypsum is reformed.
Paper covered wallboard is a popular building material. However, for certain building applications it would be advantageous to provide a gypsum panel that did not rely on paper surface sheets for strength and other properties. Several prior art fiber-reinforced gypsum panels are as follows:
U.S. Pat. No. 5,320,677 to Baig, which is incorporated by reference herein in its entirety, describes a composite product and a process for producing the product in which a dilute slurry of gypsum particles and cellulosic fibers is heated under pressure to convert the gypsum to calcium sulfate alpha hemihydrate. The cellulosic fibers have pores or voids on the surface and the alpha hemihydrate crystals form within, on and around the voids and pores of the cellulosic fibers. The heated slurry is then dewatered to form a mat, preferably using equipment similar to paper making equipment, and before the slurry cools enough to rehydrate the hemihydrate to gypsum, the mat is pressed into a board of the desired configuration. The pressed mat is cooled and the hemihydrate rehydrates to gypsum to form a dimensionally stable, strong and useful building board.
U.S. Pat. No. 6,197,235 to Miller et al., which is incorporated herein by reference in its entirety, discloses a method for texturing gypsum fiber panels and producing surface textured panels, edge tapers, and deeper patterned wainscot-type panels, involving the use of a flexible die with a textured surface. The die is pressed onto the panel in its slurry state just after the onset of an exothermic rehydration reaction. Partial hydration and setting occur during pressing by the die to form a textured mat. The mat is removed from contact with the die at a point along the rehydration temperature curve about at or less than one-half of the rise to the greatest rehydration temperature.
U.S. Pat. No. 6,605,186 to Miller, which is incorporated herein by reference in its entirety, discloses a headbox for use in a water felting process for gypsum/fiber board production including a housing and two rotating horizontal distribution rolls. The housing has curved sections shaped to respectively conform to the outer cylindrical surface of the distribution rolls. Each curved section is respectively closely spaced to a portion of the outer cylindrical surface of both distribution rolls.
Scrim embedment in panel products has been used to improve physical properties. U.S. Pat. No. 6,508,895 to Lynn et al., incorporated herein by reference in its entirety, discloses a gypsum/fiber board having improved impact resistance produced by mixing predetermined amounts of fibers, calcined gypsum and water to form a mixture; and embedding a reinforcing mesh in a layer of the mixture over the upper surface of a forming belt; and forming a board composed of bonded fibers and gypsum with the mesh embedded in the surface of the board.
Previous attempts have used scrim embedment downstream of a headbox a headbox. Scrim fed downstream of the headbox limits control of embedding, caused buildup problems on scrim embedment equipment and adversely affected formation properties in the forming pond. In this process, the scrim is fed above the headbox, down into the pond and then under a rod which assists in placing the scrim at a desired depth.